A convenient way of coupling such bars to other structural members involves the use of hammerhead-type connectors which are insertable through the slot into the channel and, upon rotation through 90.degree., are positively held against removal without further rotation. Not so positively determined, however, is the location of the connector along the bar since the hammerhead in its working position can freely slide in the channel unless firmly clamped in place. Thus, in cases where a longitudinal shifting of the connector is to be avoided, it has heretofore been necessary to provide supplemental fastening means for immobilizing the connector in a selected position.